1. Field of the Invention
This invention relates to novel polyimides having excellent solubility in organic solvents and a low softening point, thermosetting resin compositions containing the polyimides and permitting low-temperature molding, extrusion or forming, molded, extruded or otherwise formed articles of the thermosetting resin compositions, such as sheet-like adhesives, as well as a production process of the novel polyimides.
2. Description of the Related Art
Polyimides have excellent heat resistance but many of them have inferior formability, for example, moldability due to their insolubility and infusibility. To form a polyimide into a film or the like, it has been the conventional practice to form a film with a varnish of its precursor, i.e., a polyamic acid in an organic solvent and then to imidate the polyamic acid. It is however necessary to store the varnish at a low temperature because the molecular weight of the polyamic acid is lowered by hydrolysis. This conventional practice is accompanied by further problems. The solvent employed is highly hygroscopic so that a resin tends to precipitate due to absorption of moisture. Upon imidation subsequent to the formation of the varnish into a film or the like, water is formed through condensation, leaving voids in the article so formed. The conventional process cannot be applied to base materials having low heat resistance, because it requires a temperature as high as at least 300.degree. C. upon imidation.
With a view toward overcoming these problems, polyimides soluble in organic solvents have been developed. For example, polyimides obtained from 4,4'-methylenebis(2,6-dialkylanilines) and acid dianhydrides such as biphenyltetracarboxylic dianhydride and pyromellitic dianhydride, as disclosed in U.S. Pat. Nos. 4,851,505 and 4,935,490, are soluble in N-methylpyrrolidone and m-cresol. These polyimides, however, all have a glass transition point higher than 400.degree. C. so that high temperatures are indispensable for their molding, extrusion or forming.
Resin compositions, each of which comprises a polyamic acid or polyimide and a polymaleimide, have found a wide variety of utility because they exhibit high strength, toughness and heat resistance when cured. For example, they are used as adhesives or varnishes for the fabrication of printed circuit boards or prepregs. These prepregs are in turn employed for the fabrication of printed circuit boards.
For example, U.S. Pat. No. 4,362,826 discloses a thermosetting resin composition which comprises a bismaleimide compound and a polyamic acid obtained from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride. By way of example, it is disclosed that this composition can be formed into a cured film (a film carried on a base material) by casting it on a base material such as a copper foil and then heating and drying the same.
A resin composition formed by adding a bismaleimide compound to a polyamic acid as described above, however, requires not only to react a polymaleimide but also to imidate the polyamic acid upon curing because of the use of the polyamic acid. It is hence necessary to heat the resin composition to a temperature of 300.degree. C. or higher upon curing. This makes it impossible to apply the resin composition to a base material which has low resistance to heat. Further, water is formed through condensation as the imidation of the polyamic acid proceeds. Occurrence of voids cannot therefore be ignored although this problem is somewhat reduced for the existence of the polymaleimide. Occurrence of such voids causes a significant reduction in bonding force especially when the composition is cured while being interposed between a Kapton.RTM. film (polyimide film) and a metal foil such as a copper foil in order to bond them together. It is also well-known to those skilled in the art that a polyimide obtained by imidating a polyamic acid as disclosed in U.S. Pat. No. 4,362,826 is insoluble in organic solvents and is infusible. In addition, U.S. Pat. No. 4,362,826 discloses no specific examples of polyimides soluble in organic solvents.
Examples of compositions each of which comprises a polyimide and a polymaleimide include those disclosed, for example, in U.S. Pat. No. 3,842,143. Each composition disclosed in this patent is formed by adding a polymaleimide to a polyimide of a low molecular weight which has been obtained by reacting a tetracarboxylic dianhydride with an excess amount of a diamine and contains an amino group at each end. When such a low m.w. polyimide is employed, however, a film obtained from the resulting composition does not have flexibility and cannot be used as a self-supporting film. U.S. Pat. No. 3,842,143 contains neither examples of compositions having flexibility sufficient to provide self-supporting films nor examples of polyimides therefor.
Further, Japanese Patent Application Laid-Open (Kokai) No. SHO 62-30112 discloses that a composition of a polyimide--which has been obtained using 3,3', 4,4'-benzophenonetetracarboxylic dianhydride, bis[4-(3-aminophenoxy)phenyl]sulfone and, optionally, 2,4-diaminotoluene--and a polymaleimide has good film formability and a film so formed shows excellent bonding property. It is also disclosed that the polyimide is soluble in a mixed solvent of toluene and m-cresol and also in N,N-dimethylformamide (DMF). However, the polyimide disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 62-30112 is not soluble in a mixed solvent of toluene and DMF. Further, the above composition is fusible, can be molded, extruded or otherwise formed at relatively low temperatures ranging from 250.degree. C. to 275.degree. C., and exhibits superb bonding property. At forming temperatures of 230.degree. C. and lower, its characteristic properties are reduced.
As is evident from the foregoing, there has been known to date neither a composition which contains a polyimide and a polymaleimide, shows excellent properties at substantially low molding, extruding or forming temperatures and has excellent film formability nor a polyimide for such a composition.
As insulating adhesives for electronic materials, adhesives such as acrylic, phenolic, epoxy and polyimide adhesives are known. Further, hot-melt adhesives are also employed for such applications.
Acrylic, phenolic and epoxy adhesives are excellent in bonding property but are inferior in heat resistance. Hot-melt adhesives require a high bonding temperature around 300.degree. C. so that limitations are imposed on bondable materials and heating apparatuses. Because of their hot-melt property, the adhesives become soft when exposed to high temperatures even after bonding, so that the hot-melt adhesives cannot retain sufficient strength. In addition, conventional polyimide adhesives require a temperature and pressure as high as 275.degree. C. and 5 MPa for bonding.